1. Field of the Invention
The invention relates to the continuous casting of metals, particularly steel, in the form of slabs or of any other similar elongate flat product.
2. Description of Related Art
It relates more precisely to the improvement in the quality of the cast products by controlling the configuration of the convective movements of the cast metal within the mold.
At the present time it has been definitely accepted, without yet being able for the causal link to be explained, that the manner in which the convective movements of the molten metal in the mold are organized is a key factor in determining the quality of the products obtained, both as regards the formation of the very uniform and regular solidification shell around the perimeter of the mold and as regards the surface and sub-shell cleanliness (incrustations of slag, pits, blisters or level of internal cleanliness as regards level inclusions).
What is also known is the importance assumed by the development, as soon as they enter the casting space, of the streams of liquid metal arriving in the mold via the lateral outlet ports of the submerged nozzle that feeds the system with metal to be cast.
Mention may be made in this regard, among others, of the article by P. H. Dauby, M. B. Assar and G. D. Lawson “Voyage dans une lingotiere de coulde continue. Mesures laser et electromagnetigues de l'hydrodynamigue de l'acier” [Journey through a continuous casting mold. Laser and electromagnetic measurements of the hydrodynamics of the steel] published in Revue de metallurgie, April 2001, Vol. 4, pages 353–356 and the publication by D. Gotthelf, P Andrzejewski, E. Julius and H. Haubrich “Mold flow monitoring—a tool to improve caster operation” at the 3rd European Continuous Casting Conference in Madrid (Spain) in 1998, pages 825–833.
As these documents rightly stress, there are overall three types of liquid steel flow within the mold: the “single roll” configuration and the “double roll” configuration, which are stable modes, and an unstable random-type flow specific to transient regimes in the casting operation.
The latter flow may be described schematically as being an erratic alternation of the “single roll” and “double roll” modes resulting from the momentary and uncontrollable dissymmetries of the flows between the two half casting spaces on either side of the nozzle that are due in particular to perturbations, even minute ones, in the energy at the outlet ports of the nozzle, such as for example, differential variations in the flow rate of antiblocking argon between the two ports.
However, the two aforementioned stable flow modes are themselves more explicit. They are illustrated in FIGS. 1A and 1B appended at the end of this specification. These figures show the stabilized pattern of the paths of the principal currents in a vertical plane passing through the casting axis and parallel to the two long walls of a continuous casting slab mold. The “single roll” mode (FIG. 1A) essentially results, as may be seen, in the fact that the jets of metal 1, as soon as they leave the ports 2 of the nozzle 3 are directed somewhat upward, toward the free surface (or meniscus) 4 of the metal poured into the mold. At this point they travel the width of the half casting space in which each develops by hugging the long walls of the mold until reaching the short end walls 5. It will be recalled, if necessary, that these short mold walls, also called “closure walls” are mounted in line with the end of the long mold walls so as to ensure continuity of the internal periphery of the mold and therefore to seal the casting space. Upon reaching the short mold wall, each jet 1 is then generally reflected downward in the direction of extraction of the cast product, indicated by the bold vertical arrow in the middle of the figure. Of course, the precise mapping of the velocities is much more complex. Many current lines, such as 6, follow paths that are more typically parabolic because of the overall downward extraction movement, but schematically it is indeed this general shape as an upwardly spouting source that is very noticeable when the “single roll” mode is observed in a simulator or in a mock-up.
In contrast, in the “double roll” mode (FIG. 1B), each jet 1, arriving in the mold via the nozzle 3, leaves the ports 2 overall horizontally and thus propagates towards the short mold walls 5, in which everything occurs as if the impact had divided the jet into two currents, a main current 8 reflected downward and a secondary current 7 reflected upward toward the meniscus 4, and at this point the secondary current then travels the half casting space in the opposite direction, this time from the short mold wall 5 toward the nozzle 3. Here again, the actual mapping is much more complex, but it is indeed this overall image in the form of “butterfly wings” that strikes the observer when looking at the screen of a modeler or a mock-up operating in “double roll” mode.
Advances in our understanding and the accumulation of experimental data now make it possible to know, quite well and how, depending on the relevant casting parameters, one or other of the two aforementioned flow modes becomes stable or virtually stable. Without entering into details, which would however be unnecessary and superfluous for comprehension of the invention, it may simply be stated that the larger the width of the slabs cast, and likewise the lower the extraction rate during casting, the more the flow field is in the “single roll” configuration, and vice versa as regards the “double roll” configuration.
It should be pointed out that the operator of the continuous caster does not in general have at his disposal the means to determine the stable flow mode of the metal within his mold. Moreover, it has to be said that very often this is of really no concern to him since, in any case, he would not know how or be able to change the casting aspect ratio or the rate of extraction, which parameters are set by the order book and the material flow within the plant.
However, recent studies by the Applicant have confirmed, if not demonstrated, the existence of explicit causal links between the defects in products resulting from the casting on the one hand (versus the disappearance of these defects) and, on the other hand, the configuration of the convective movements of the liquid metal in the mold. Thus, the origin of the observed quality defects turns out to be due not only to flows of the unstable type, which one might have suspected, but also the stable configuration in “single roll” mode.